1. Field of the Present Invention
The present invention relates to an objective lens used for an optical pickup and an optical disc apparatus that performs recording and/or reproducing information on an optical recording medium such as an optical disc and an optical pickup and an optical disc apparatus using the objective lens.
2. Description of the Related Art
In the related art, as a recording medium for information signals, there have been optical discs such as a CD (Compact Disc) using a light beam having a wavelength of about 785 nm and a DVD (Digital Versatile Disc) using a light beam having a wavelength of about 660 nm, capable of implementing recording with a higher density than that of the CD. In addition, as the optical disc, there has been a high density recordable optical disc (hereinafter, referred to as a “high density recording optical disc”) which performs recording and reproducing signals using a light beam having a wavelength of about 405 nm emitted from a blue violet semiconductor laser, capable of implementing recording with a higher density than that of the DVD. As the high density recording optical disc, there has been proposed a structure such as BD (Blu-ray Disc (Registered Trade Mark)), in which a cover layer (protective layer) for protecting a recording layer, on which signals are recorded, is configured to have a small thickness.
An optical pickup has been used to perform recording information signals on the aforementioned optical disc such as a CD, a DVD, and a BD and to perform reproducing the information signals recorded on the optical disc. The optical pickup includes a light source that emits a light beam, an objective lens that condenses the light source emitted from the light source on the recording layer of the optical disc, a fixed optic system that guides the light beam from the light source to the objective lens, and the like.
In the optical pickup in the related art, at the time of assembling and adjusting the objective lens that condenses the light beam on the recording layer of the optical disc, the objective lens may be arranged to be tilted with respect to an ideal optical disc (hereinafter, referred to as a “reference disc”) having no warpage, which is used as a reference. In other words, at the time of assembling and adjusting, an optical axis of the light beam that is guided from the fixed optic system to the objective lens may be arranged to be tilted with respect to an optical axis of the objective lens. In this manner, the objective lens is assembled to be tilted, so that a coma aberration of the objective lens and a coma aberration caused by other parts of the optical pickup, which are included in the fixed optic system and the like, can be cancelled to be reduced. This process has been generally used so as to reduce the coma aberrations. In addition, in the case of reducing the aforementioned coma aberration, the objective lens is disposed to be tilted.
It has been known that the coma aberration caused by a relative slope between the objective lens and the optical disc is proportional to the cube of numerical aperture (NA) of the objective lens and the cover layer thickness and proportional to the reciprocal of wavelength. Therefore, in the case of an optical pickup for a BD in which a high NA and a short wavelength is combined, the problem of coma aberration may easily occur. In order to solve the problem, the aforementioned lens adjusting method has been generally used.
In order to improve mass-productivity and to implement light weight, the objective lens for the optical pickup is preferably made of a plastic (synthetic resin) as a substitute for a glass that has been widely used in the related art. Recently, a plastic having an excellent light resistance to a blue laser has been developed. Therefore, a plastic objective lens has been considered to be employed. In general, since a glass objective lens has a small change in refractive index according to a change in temperature, a spherical aberration amount of an optic system caused by the change in temperature is small, so that behaviors due to the change in temperature have no problem.
However, since a plastic lens generally used is greatly influenced by the change in temperature, a ratio of deformation or change in refractive index is larger than that of a glass lens. On the other hand, since the optical pickup is used under various environments, the optical pickup is necessary to normally operate in a predetermined temperature range. Due to the change in refractive index associated with the change in environment temperature, the spherical aberration occurs. Therefore, the change in spherical aberration caused by the change in refractive index, where has no problem in the optical pickup apparatus in the related art, is not a negligible amount in the configuration of combining a laser having a short wavelength and an objective lens having a high NA.
The changing spherical aberration caused by the change in refractive index can be cancelled to be reduced by driving a collimator lens included in an optical pickup optic system in the optical axial direction. However, if the collimator lens is moved so as to reduce the spherical aberration, the problem of a change in incidence magnification of the light beam to the objective lens occurs. In other words, the incidence magnification of the light beam to the objective lens is greatly changed substantially in proportion to an amount of the changing spherical aberration.
Herein, if the incidence magnification to the objective lens is changed, a deviation from an aplanatic design occurs, and the amount of the coma aberration caused by the tilting of the objective lens is changed. Hereinafter, in the case where the coma aberration amount ΔWLT [λrms] occurs at the time of tilting the objective lens by Δθ [deg] (hereinafter, referred to as a “lens tilt”), a sensitivity defined by ΔWLT/Δθ is referred to as a “lens tilt sensitivity”. In addition, the incidence magnifications causing the optimum spherical aberration are different between the high and low environment temperature cases, so that the lens tilt sensitivities are different between the high and low temperature cases.
As described above, in the case where the objective lens is adjusted to be fixed in the tilt direction, the coma aberration at the temperature such as the room temperature at the time of adjusting is reduced. In the case where the lens tilt sensitivity is changed due to the change in environment temperature in the state, there is a problem in that the coma aberration amount that is to be corrected at the time of adjusting is different from the coma aberration amount that really occurs due to the tilting of the objective lens. In other words, the lens tilt sensitivity is different due to the change in environment temperature, so that the real tilt angle is different from the optimum adjusting angle. Therefore, there is a problem in that the coma aberration occurs to the extent that the coma aberration influences the quality of signal. In addition, the behavior of the change in lens tilt sensitivity is different between the high and low temperature in the change in environment temperature, so that the lens tilt sensitivity may be very low. In this case, there is a problem in that the coma aberration caused by the change in temperature or the coma aberration caused by the warpage of the optical disc may not be cancelled by the lens tilt. In addition, there is a problem in that the aforementioned coma aberration has an adverse influence on a quality of signal.    Japanese Unexamined Patent Application Publication No. 2008-112575 is an example of related art.